In the following description, assume that ‘legacy system’ means a predefined system in advance and that ‘evolved system’ means a system evolved from the legacy system or a newly defined system.
‘Legacy support’ means to support a legacy system in transmitting and receiving relations with an evolved system. And, assume that the following two conditions are met in a broad sense.
1) First of all, a legacy base station (hereinafter abbreviated BS) and a legacy mobile station (hereinafter abbreviated MS) are able to transmit and receive signals without being affected by an evolved system. And, signal transmission and reception can be performed between the legacy BS and an evolved MS.
2) Secondly, a BS available for both legacy and evolved systems are able to transmit and receive signals to/from a legacy MS and an evolved MS both.
For clarity and convenience of the following description, 3GPP LTE (3rd generation partnership project long term evolution) system is assumed as a legacy system and 3GPP LTE-advanced (hereinafter abbreviated LTE-A) system is assumed as an evolved system.
After a basic configuration of system has been established, if a new service is added, a problem of legacy support is always caused. Specifically, a system needs to be evolved in a direction for not affecting performance of a legacy system. For this, an important control channel used for a legacy system should be protected. Yet, it is not preferable that the protection for the legacy system degrades performance of a new technical component.
FIG. 1 is a diagram of a radio FDD (frequency division duplexing) frame structure in 3GPP LTE system in case of a normal cyclic prefix (CP).
Referring to FIG. 1, in the radio frame structure, one radio frame (e.g., a length of the radio frame is 10 ms) is constructed with total 10 subframes (e.g., a length of each subframe is 1 ms). And, usages of some of the subframes are specified. In particular, usages of 0th, 4th, 5th and 9th subframes are specified as follows.
First of all, the 0th and 5th subframes (i.e., subframe 0 and subframe 5 shown in FIG. 1) are configured to carry primary synchronization channel (PSCH) and secondary synchronization channel (SSCH) for a synchronization signal, respectively. The 0th subframe is configured to carry a physical broadcast channel (PBCH) as well as the synchronization signal. Therefore, the 0th subframe in a system is configured to carry the SSCH, PSCH and PBCH and the 5th subframe is configured to carry the SSCH and PSCH. A 4th subframe (represented as subframe 4 in FIG. 1) is configured to carry SIB (system information block) information. And, a 9th subframe (represented as subframe 9 in FIG. 1) is defined as a special subframe for unicast.
Particularly, in each of the 0th and 5th subframes, 5th OFDM (orthogonal frequency division multiplexing) symbol is an OFDM symbol for carrying SSCH and a 6th OFDM symbol is an OFDM symbol for carrying PSCH. In the 0th subframe, 7th to 10th OFDM symbols are OFDM symbols for carrying PBCH.
FIG. 2 is a diagram for explaining a principle of HARQ (hybrid automatic repeat request).
Referring to FIG. 2, HARQ is the hybrid technique generated from combining ARQ technique of MAC layer and channel coding scheme of physical layer together, as inferred from the name of the HARQ. In HARQ, since an initially transmitted packet, which is erroneous, is a signal P1A having a prescribed information size, it is stored rather than discarded until a retransmitted signal is received. Soft combining is then performed on the stored signal together with the retransmitted signal P1B. Alternatively, a signal is decoded by a different method using the initially transmitted signal and the retransmitted signal together. In FIG. 2, the P1A and the P1B are rendered from the same information bit, i.e., the same channel encoder input packet P1 and are transmission packets identical to each other or slightly different from each other, respectively. And, P2A indicates a packet rendered from a new channel encoder input packet P2.
In the above described frame structure, a physical structure of data communication uses HARQ but its period is defined as Hp (e.g., 8 ms). This means a structure that Hp processes can be carried on subframes by being interlaced, respectively. Hence, it can be observed that they can be repeated with Hp subframe interval.
On the contrary, if a transmit unit of a radio frame transmitted by a base station is set to Rp, the radio frame is repeatedly transmitted by a period Rp (e.g., 10 ms) that is the transmit unit of the radio frame.
Under this circumstance, if a subframe of a specific type is designated, how to configure subframe designating information of the specific type causes a problem. For instance, the subframe of the specific type can include one of MBSFN (Multimedia Broadcast multicast service Single Frequency Network) subframe, relay subframe, blank subframe, Positioning subframe, LTE-A subframe, and the like.